The present invention relates to systems and methods for displaying flight information representative of a deviation from a specified four-dimensional flight path. Particularly, but not exclusively, the invention relates to a cockpit display for enabling pilots to arrive at a particular location at a particular time and to achieve the necessary changes to aircraft speed to achieve a preferred ground speed.
Flight paths are conventionally calculated in three dimensions, i.e. longitude, latitude and altitude. Future air traffic control systems are expected to specify a further dimension to flight paths, namely time. In other words, such air traffic control systems will not only provide a path that an aircraft must follow, but will also specify the time at which an aircraft must be at each position along the path. This will allow air traffic control to relax the requirement for separation between aircraft and thereby allow more efficient use of airspace.
Conventional cockpit displays display only absolute airspeed, a target for absolute airspeed, and an airspeed trend arrow. The latter indicates the aircraft's predicted speed after a fixed time period (typically, six to ten seconds) in dependence upon the current rate of change of airspeed. Such cockpit displays are suited to the presently used air traffic control systems, which may specify absolute speed commands in the form of target airspeeds. However, they do not provide the flight crew with information that is directly relevant to the task of accurately following a four-dimensional flight path.
Airspeed measurements are not directly comparable with ground speed values, since airspeed is measured relative to the body of air that the aircraft is travelling through. This body of air can itself be moving relative to the ground. The movement of the body of air is characterised by wind speed and wind direction, each of which can vary over time. Therefore, ground speed is equivalent to the magnitude of the sum of the vector defining the aircraft's airspeed and the vector defining the wind speed. Typically, the vertical direction is not considered in this comparison since it has a negligible effect. In view of the time varying nature of airspeed relative to ground speed, the display of airspeed is not appropriate for flight crews trying to accurately follow a four-dimensional flight path.